In rotational molding or rotomolding, the product or molded object is formed inside a closed mold or cavity while the mold is rotating bi-axially in a heating chamber. There are typically four steps, or stages, in a rotational molding process: loading, molding (or curing), cooling, and unloading. In the loading stage, either liquid or powdered plastic, which may be a thermoplastic, is charged into a hollow mold. The mold is closed, rotated about two orthogonal axes, and moved into a heating chamber or oven for the molding or curing stage. In the oven, heat penetrates the mold, causing the plastic to melt, adhere to, and sinter onto the mold surface. The mold continues to rotate during heating, and the plastic gradually becomes distributed evenly on the mold walls through gravitational force. As the cycle continues, the plastic melts completely, forming a homogeneous layer of molten thermoplastic on the interior surfaces of the mold. While continuing to rotate, the mold is moved out of the oven to a cooling area or chamber for the cooling stage, where the plastic is cooled to the point that the molded object will retain its shape. During cooling, the molded object typically shrinks away from the mold. In the unloading stage, rotation of the mold stops, and the mold is opened to remove the molded object.
Examples of molds and processes for rotational molding are disclosed in U.S. Pat. Nos. 4,049,767, 4,146,565, 4,247,279, 4,980,112, 5,039,297, and 5,094,607. Further examples and descriptions of molds and processes for rotational molding are provided in Glenn L. Beall, Rotational Molding: Design, Materials, Tooling, and Processing (Hanser/Gardner Publications, Inc. 1998) and in Paul Nugent, Rotational Molding: A Practical Guide (2001). The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes.